Add a lens for /etc/shadow file format

and wire-up its test to the runner's list in Makefile.am

Also add a mock /etc/shadow in tests/root
Add a lens for /etc/gshadow file format

and wire-up its test to the runner's list in Makefile.am

Also add a mock /etc/gshadow in tests/root
Add a little bit of ducumentation

for both the shadow and the gshadow lenses
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